1. Field of the Invention
The present invention is in the field of ceramic capacitors and relates more particularly to a trimable ceramic capacitor, i.e. to a capacitor which may be adjusted to precise tolerances as required in certain applications. The invention is further directed to an arc resistant capacitor which may also be trimmed if desired.
2. Prior Art
As conducive to an understanding of the present invention, it is to be noted that in many applications it is necessary to provide monolithic ceramic capacitors which are tuned or adjusted to precise tolerance limitations. In the normal manufacturing procedures for ceramic capacitors, it is virtually impossible to achieve on a repeatable basis ceramic capacitors of the necessary tolerance accuracy. This is true, because the capacitance will depend to a great degree on such factors as the thickness of the green ceramic layers, the processing parameters employed, and the composition of and particle size of the ceramic granules in any given batch which is processed.
In view of the aforementioned difficulties of obtaining ceramic capacitors of precise values, it is conventional to manufacture the capacitor to approximate tolerance ranges and to adjust the capacitance by reducing the same after the capacitor has been formed. Representative examples of patents teaching such post-formation tolerance adjustments are as follows:
3,394,386; Weller et al PA1 3,456,170; Hatch PA1 3,688,361; Bonini PA1 3,694,710; Kirschner PA1 3,898,541; Weller PA1 4,074,340; Leigh PA1 4,190,854; Redfern PA1 4,466,045; Coleman PA1 4,467,393; Kupfer PA1 4,470,096; Guertin
In general, the adjustment procedures involved in respect of such capacitors involve eroding or abrading away increments of the dielectric and upper layer or layers of electrode to progressively reduce the overlapping area of dielectric and electrode until the capacitance is reduced to the desired value. The eroding may be effected while the capacitance is continuously measured.
It has been proposed, in order to facilitate tolerance adjustments of ceramic capacitors to apply a surface or exposed electrode atop the outermost layer of ceramic and to effect final tolerance adjustments by eroding portions of the exposed electrode layer only, rather than requiring eroding of the more durable ceramic components. Attempts to employ surface electrode erosion for capacitance adjustment have heretofore proven commercially unfeasible. For reasons which were heretofore unexplained, capacitors which were modified to a specific capacitance value were found over a period of time to shift in value rendering the same unsuitable for use. Additionally, capacitors having surface electrode layers were highly subject to arc over problems, i.e. electrical discharges between margins of the surface electrode and end termination of the capacitor.